The present invention relates to a reference electrode and more particularly to a reference electrode apparatus for use in connection with a tube sheet for a condenser or heat exchanger.
Condensers and heat exchangers used for power plants and in the oil refining industry include large tube sheets into which tubes are inserted. As these condensers and heat exchangers are used in environments in which metallic parts can corrode, it is a constant goal to prevent such corrosion and thereby extend the life of the tubes and tube sheets.
One means of preventing corrosion of metallic parts immersed in a corrosive electrolyte, such as seawater, is through cathodic protection which utilizes an electric current flowing between an anode and the surface to be protected which is the cathode. The current may be generated naturally by using an anode, commonly referred to as a sacrificial anode, which is more electronegative than the cathode. Alternatively, the current may be supplied by an electrical device commonly referred to as an Impressed Current Cathodic Protection System. If an excess of current flows between the anode and cathode, hydrogen can be generated at the cathode which can embrittle certain susceptible metals, such as titanium or ferritic stainless steels, or cause the blistering of any paint films which have been applied to the surface. On the other hand, if an insufficient amount of current is applied, the surface will not be fully protected against corrosion. Therefore, an optimum level of protection current exists, and this optimum level can be determined by measuring the potential of the protected surface as this surface will establish a certain optimum potential when the proper level of protection current has been applied. Overprotected surfaces will be more negative and underprotected surfaces less negative than the optimum protection potential.
In order to measure the potential of the protected surface, reference electrodes are immersed in the electrolyte and the potential between the reference electrode and the protected surface is measured with an electrometer or other similar instrument. This measurement can then be used to control the current by manually or automatically adjusting the protection current level to keep this potential as close to the optimum as practical. Reference electrodes used for this purpose include: silver/silver chloride, calomel, zinc and copper/copper sulphate, in addition to other types.
A reference electrode should be positioned as close to the surface to be measured as practical. Increasing the distance between the reference electrode and the surface to be measured decreases the accuracy of the measurement by providing interference from the resistance of the electrolyte as well as by masking local variations in potential which can be indicative of areas of the cathode which are being under or over protected. While minimizing the distance between the reference electrode and the cathode surface is possible in laboratory environments, it has not been practical to do so in industrial applications such as shell and tube heat exchangers and condensers.
In the case of a cathodically protected tube sheet 12 shown in FIG. 1, the reference electrode 16 will frequently be mounted on the wall of a water box 18 and located several feet away from the surface of the tube sheet 12. A principal reason why electrodes are positioned several feet from the surface of the tube sheet is that there is no simple and reliable way to permanently and rigidly mount a reference electrode directly on the surface of the tube sheet.
It is therefore a principal object of the present invention to provide a reference electrode for use in conjunction with a tube sheet for a shell and tube heat exchanger or condenser.
It is a further object of the present invention to provide a reference electrode which may be positioned in close proximity to a tube sheet for a tube heat exchanger or condenser.
Still another object of the present invention is to provide a reference electrode that can be mounted to tube plugs already inserted in an end of a tube inserted to a tube sheet.
Yet another object of the present invention is to provide a reference electrode that can be permanently and rigidly mounted on the surface of the tube sheet.